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John Knaff , NOAA/NESDIS/ StAR , RAMMB, Fort Collins, CO, USA PowerPoint PPT Presentation


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Central Pressure – Maximum Wind Relationships in Tropical Cyclones using operationally available information. John Knaff , NOAA/NESDIS/ StAR , RAMMB, Fort Collins, CO, USA Joe Courtney, Australian BoM , Perth, WA, Australia

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John Knaff , NOAA/NESDIS/ StAR , RAMMB, Fort Collins, CO, USA

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John knaff noaa nesdis star rammb fort collins co usa

Central Pressure – Maximum Wind Relationships in Tropical Cyclonesusing operationally available information

John Knaff, NOAA/NESDIS/StAR, RAMMB, Fort Collins, CO, USA

Joe Courtney, Australian BoM, Perth, WA, Australia

Ray Zehr, NOAA/NESDIS/StAR, RAMMB, Fort Collins, CO, USA (retired)


Determining central pressure cp

Determining Central Pressure (CP)

  • Environmental Pressure (Penv; i.e., Boundary Condition)

  • Pressure Deficit

    • Determined by the integral of the wind field

, where

r is the radius

ρ is the density

Vt is the tangential wind

̅ represents azimuthal averaging


Some implications

Some Implications

  • Larger storms when other variables are held constant (wovhc) have lower CP

  • Smaller (Larger) radius of maximum wind wovhc implies lower (higher) CP

  • Lower Penvwovhc implies lower CP

  • Higher latitude wovhc implies lower CP


Examples earl sept 1 18utc

Examples (Earl Sept 1 18UTC

IR image

Azimuthally averaged Vt

P600km = 1012.1 hPa

ΔP = -68.6 hPa

CP = 943.5

Vt = 45.8 ms-1, 89 kt


Examples darby june 26 06 utc

Examples (Darby June 26 06 UTC)

IR image

Azimuthally averaged Vt

P600km = 1011.7 hPa

ΔP = -52.9 hPa

CP = 958.8

Vt = 44.3 ms-1, 86.1 kt


Operational challenges

Operational Challenges

  • Historical wind-pressure relationships target the mean relationship and don’t account for different…

    • Environments

    • Steering

    • Sizes

    • Latitudes

  • Intensification rates vary

  • Observational data are limited, sparse, & latent.

  • CP is often required for advisories and forecasts


New methods

New Methods

  • Use operationally available information to quantify

    • Environmental Pressure (Penv)

    • TC size (S)

    • Intensification rate

    • Maximim winds, 1-minute max sustained (Vmax)

    • Latitude (φ)

    • Translation Speed (c)

  • Determined the most important factors (i.e., Penv, Vmax, φ,c, S)

  • Develop universal techniques to estimate CP from maximum wind and vise versa.

    More reading:

    Knaff, J.A., and R.M. Zehr, 2007: Reexamination of Tropical Cyclone Wind-Pressure Relationships. Wea Forecasting,22:1, 71–88. 

    Knaff, J.A. and R.M. Zehr, 2008:  Reply to Comments on "Reexamination of Tropical Cyclone Wind-Pressure Relationship." Weather and Forecasting, 23:4, 762-770.

    Courtney, J., and J.A. Knaff, 2009: Adapting the Knaff and Zehr Wind-Pressure Relationship for operational use in Tropical Cyclone Warning Centres. Australian Meteorological and Oceanographic Journal, 58:3, 167-179.


Factor 1 storm relative intensity i e combining v max and c

Factor #1: Storm relative intensity (i.e., combining Vmaxand c)

Accounting for translation

Example (Vmax =100kt TC)

  • Use Schwerdt et al. (1979) asymmetry factor (a)

    • 1.50c0.63 [kt]

    • 1.26c0.63 [ms-1]

    • 1.88c0.63 [kmh-1]

      Define: storm relative intensity


Factor 2 environmental pressure p env

Factor #2: Environmental Pressure (Penv)

Estimating Penv

Comments

Cumulative term so…This is a factor that just needs to be representative of the environment.

Standardizes seasonal and inter basin differences

ranged from 1004 to 1026 hPa with an average of 1014 in the Atlantic dataset

1002 to 1016 with a 1009 average in the West Pacific.

  • Knaff and Zehr (2007): Azimuthally averaged MSLP at r=900 km (10 degrees) from global analyses

  • Courtney and Knaff (2009): Pressure of outer closed isobar method.


Factor 3 tc size s

Factor #3: TC Size (S)

Quantifying TC Size

Climatology of Size

  • Knaff & Zehr: Calculate the tangential wind at r=500km (V500) from global analyses

  • Courtney & Knaff: Estimate V500from the non-zero average of the gale radii, where V500= R34/9 – 3

  • Must account for climatological size variations (V500c) due to

    • Intensity

    • Latitude

Rmax is valid just for the wind profile estimation

and is generally too large when compared to

observations (i.e., not a good estimate for RMW)

More reading:

Knaff, J.A., C. R. Sampson, M. DeMaria, T. P. Marchok, J. M. Gross, and C. J. McAdie, 2007: Statistical Tropical Cyclone Wind Radii Prediction Using Climatology and Persistence, Wea. Forecasting, 22:4, 781–791.


V 500c

V500c


Putting it all together

Putting it all together

  • Courtney & Knaff:

  • Caveats:

  • the minimum value of S should be limited to a value between 0.4 and 0.1

  • estimates are sensitive most to poor estimates of S (i.e. R34) and Vsrm


Simple example

Simple Example

Penv: POCI=1009, Penv=1011 hPa

Latitude: φ=20

Translation: c=10 kt

Intensity: Vmax = 75 kt

Vsrm= 75 – 6.40 = 68.6 kt

Size climo: x=.528,Rmax=54.65,V500c=23.3

Size: R34= 170, 120, 130, 180

averaged R34=150

V500=13.66

S=13.66/23.3=0.586

ΔP:ΔP=-35 hPa

CP: CP=976 hPa


Validation

Validation

Dvorak Atantic

Using Courtney & Knaff

Courtesy of C. Landsea (NHC)


Sensitivities

Sensitivities


Shortcomings

Shortcomings

  • Large errors (10-30 hPa) can occur when small or multiple radii of maximum winds occur

    • Currently lack a reliable estimate for all intensities

    • Currently lack an empirical correction

  • Pressure estimates only as good as the input

    • R34

    • Intensity from Dvorak, SATCON, AMSU etc each with it’s biases

    • Translation speed (tough for multiple centers and weaker poorly defined TCs


Estimating v max

Estimating Vmax

All values here are valid for 1-minute winds in units of knots

We recommend a factor between 0.88 and 0.93 for 1-minute

to 10-minute conversion

Also note that iteration is required to accurately solve since S is a function of Vmax


Additional reading references

Additional Reading References

Courtney, J., and J.A. Knaff, 2009: Adapting the Knaff and Zehr Wind-Pressure Relationship for operational use in Tropical Cyclone Warning Centres. Australian Meteorological and Oceanographic Journal, 58:3, 167-179.

Harper, B. A., J. Kepert and J. Ginger, 2008a: Wind speed time averaging conversions for tropical cyclone conditions. AMS 28th Conf Hurricanes and Tropical Meteorology, Orlando, 4B.1, April.

Harper, B. A., J. D. Kepert, and J. D. Ginger, 2010: Guidelines for converting between various wind averaging periods in tropical cyclone conditions. World Meteorological Organization, TCP Sub-Project Report, WMO/TD-No. 1555.

Knaff, J.A., and B.A. Harper, 2010:  Tropical cyclone surface wind structure and wind-pressure relationships.  Keynote #3, WMO International Workshop on Tropical Cyclones – VII, La Reunion, France, 15-20 November, 35pp.

Knaff, J.A. and R.M. Zehr, 2008:  Reply to Comments on "Reexamination of Tropical Cyclone Wind-Pressure Relationship." Weather and Forecasting, 23:4, 762-770.

Knaff, J.A., and R.M. Zehr, 2007: Reexamination of Tropical Cyclone Wind-Pressure Relationships. Wea Forecasting,22:1, 71–88.

Knaff J. A., C. R. Sampson, M. DeMaria, T. P. Marchok, J. M. Gross, and C. J. McAdie, 2007: Statistical tropical cyclone wind radii prediction using climatology and persistence, Wea. Forecasting, 22:4, 781–791.

Knaff J. A., D.P. Brown, J. Courtney, G. M. Gallina, and J. L. Beven, II, 2010: An evaluation of Dvorak technique-based tropical cyclone intensity estimates. Wea. Forecasting, in press. ; e-View doi: 10.1175/2010WAF2222375.1

Schwerdt,R. W., F. P. Ho, and R. R. Watkins, 1979: Meteorological criteria for standard project hurricane and probable maximum hurricane wind fields, Gulf and East Coasts of the United States. NOAA Tech. Rep. NWS 23, 317 pp. [Available from National Hurricane Center Library, 11691 SW 117 St., Miami, FL 33165-2149.]

Many of these are available at http://rammb.cira.colostate.edu/resources/publications.asp or upon request.


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